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Building Science Fundamentals

Understand Building Shell / Envelope Leakage As A
Function Of Pressure Difference And The Size Of Holes In
The Air Barrier

Understand Building Shell / Envelope Leakage As A
Function Of The Pressure Difference

Understand Building Shell / Envelope Leakage As A
Function Of The Size Of Holes In The Air Barrier

Using the effective leakage area, you can get a rough idea of how big the hole is you are trying to seal. The effective leakage area says that for every 1000 CFM50, there's about one square inch of leakage area in the envelope. Then compare that to how big you want the hole to be using ASHRAE standards for ventilation and the MVR. The difference between the two will tell your crew how much air leakage should be done in understandable terms.

Some examples of using this technique are below.

You find the CFM50 of a home is 2350 CFM50 with all windows closed and interior doors open. With the blower door still running at 50 Pa, disconnect the manometer and leave one tube connected to A or B. Then place the tube in a bedroom or room of choice and close the door. You record a pressure reading of 15 Pa, open the door and repeat the steps under all the doors to get pressure readings of 5 Pa, 7 Pa and 2 Pa. These numbers will give you an idea of what room has more air sealing opportunities than others with the high numbers signaling more air leakage.
You find the CFM50 of a home is 6350 CFM50 with all windows closed and interior doors open. This means that you have approximately a 6350 / 10 = 635 sq inch window open in the home. So lets find where that 635 sq inches is coming from. In the attic you discover a small 1" x 1" hole where an electrical wire goes into the home. Seal that up and you'll be down to 634 sq inch of leakage. You continue and you find the following: 10 accessible 1x1 sq in holes from electrical wires and a 12"x12" open wall cavity. Add those areas up and you can potentially seal a 22x22 sq in hole or a 484 sq inches.

Next Section

3a. Applied diagnostics and troubleshooting